1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device including a transfer step of a semiconductor element such as a thin film transistor (TFT) formed over a substrate, to another substrate.
In addition, the present invention relates to a method for manufacturing a semiconductor device including a transfer step of an integrated circuit comprising a plurality of semiconductor elements (including a thin film transistor (TFT)) formed over a substrate, to another substrate.
2. Description of the Related Art
In recent years, various technical developments have been made about a semiconductor element represented by a thin film transistor (TFT) that is formed by using a semiconductor thin film (about from several nm to several hundred nm in thickness) formed over the same substrate.
To secure characteristics of a semiconductor element, a certain level of high temperature is required in the manufacture process. At present, a significant decrease of the step temperature is realized by introducing laser crystallization into a part of the manufacture process. As a result, it becomes possible to manufacture a semiconductor element over a glass substrate, which has been impossible in the high temperature process.
However, it is extremely difficult to manufacture a semiconductor element over a flexible substrate such as plastic from a viewpoint of heat-resistance of a substrate, because much lower-temperature is necessary.
On the contrary, a method for transferring a semiconductor element over a flexible substrate such as plastic after forming the semiconductor element over a glass substrate is known as an effective method, because the method substantially can remove thermal disincentive to a substrate.
There are several proposed methods for separating a semiconductor element formed over a substrate from the substrate. For example, (1) a method that a semiconductor element is formed over a glass substrate, then the glass substrate is dissolved by etching to separate the semiconductor element alone, and then, the semiconductor element is bonded to a plastic substrate (Reference: Japanese Patent Laid Open No. 2002-184959). (2) a method that a semiconductor element formed over a substrate is separated from the substrate, and then, is bonded to a flexible substrate such as plastic (Reference: Japanese Patent Laid Open No. H10-125931).
In the case of the method (1), there is a problem of a longer process time or high material cost of an etching material or a glass substrate, although a substrate is completely separated from a semiconductor element after forming the semiconductor element over the substrate.
In the case of the method (2), it is very important to control adhesiveness and film stress. Namely, there is a problem of peeling off while manufacturing a semiconductor element, although it is easier to perform separation with lower adhesiveness and higher film stress. There is also difficulty in performing separation of a semiconductor element from a substrate after the semiconductor element is formed, although peeling-off of the semiconductor element does not occur in the middle of manufacturing the semiconductor element, if the adhesiveness is enhanced.